The overall goal of this project is to develop a novel, broad-spectrum neurosteroid therapy that will mitigate the morbidity and mortality caused by acute exposure to nerve agents and organophosphate (OP) pesticides, which are credible threat agents for military and civilian populations. Exposure to nerve agents or OP poisoning can result in persistent seizures, status epilepticus (SE), and permanent brain injury. Current anticonvulsant countermeasures (benzodiazepines) for nerve intoxication do not sufficiently protect the brain from SE, a prolonged seizure activity lasting 30 min or longer with significant neuronal injury and mortality. We propose that neurosteroids and selective drugs that enhance phasic and extrasynaptic tonic inhibition produce more effective protection against persistent SE than benzodiazepines, prevent irreversible brain injury, and extend the therapeutic window. This novel therapeutic strategy is based on the emerging molecular mechanisms of neurosteroids and also cellular changes involved in SE, a common neurotoxicity by nerve agents. Neurosteroids are the most powerful anticonvulsants against seizures induced by cholinergic agents. Since OP nerve agents cause persistent seizures and brain damage through cholinergic hyperactivation, it is proposed that neurosteroids are more effective anticonvulsants for OP nerve intoxication than benzodiazepines. We have pilot evidence that late post-exposure neurosteroid therapy can rapidly control SE and be neuroprotective after SE in pilocarpine and DFP models, suggesting the promising efficacy of neurosteroid therapy. The objective of this project is to investigate the efficacy and safety of the synthetic neurosteroid ganaxolone and its analogs as 'broad-spectrum' medical countermeasures for nerve agent and OP pesticide intoxication. This is a logical extension of our R21s for further efficacy validation, lead optimization, and IND application. The primary emphasis is to generate requisite data on the efficacy and safety of the lead candidate and submit an IND application within a 5 year period. To develop neurosteroids as superior countermeasures than diazepam, we will address three specific aims: (Aim 1): Determine the efficacy of ganaxolone against DFP- and soman-induced SE and brain damage; (Aim 2): Develop ganaxolone analogs with improved formulation pharmacokinetics and efficacy against DFP- and soman-induced SE and brain damage; and (Aim 3): Determine the preclinical safety and toxicity of ganaxolone or its lead analog and submit an IND for clinical development under the FDA Animal Rule Pathway. The project will be implemented as per the progressive go/no-go milestones plan, which includes quantitative criteria for the success of key studies focusing on three primary outcome measures: (i) anticonvulsant efficacy; (ii) neuroprotectant efficacy; and (iii) prevention of neurodegeneration. The outcome from this project will identify an effective antidote for OP intoxication and enhance readiness for emergencies.